As cellular communications become more widely used, the number of individual users and calls multiplies. Increase in cellular communications utilization magnifies the opportunity for interference between the different users on the cellular system. Such interference is inevitable because of the large number of users and the finite number of cellular communications cells (cells) and frequency bands, time divisions, and/or identifying codes (collectively referred to herein as channels, although the use of frequency bands of frequency division multiple access (FDMA), time slots of time division multiple access (TDMA), pseudo random codes of code division multiple access (CDMA), and the like may be utilized in distinguishing "channels") available.
As originally implemented, cellular communications systems have been broken down into omni-trunks where each cell was able to use each channel in a 360.degree. radius. Because of overlap in the area serviced by cells, a caller utilizing one cell in the penumbra between two cells could interfere with a caller utilizing the other cell if both were on the same channel. To avoid this interference the channel utilized by a caller in one cell would have to be disabled for any other callers in any adjacent cells. Disabling such a channel on all adjacent cells would cause many more cells than actually utilized to have the caller's channel unavailable for use by other callers. Such disabling of channels to avoid interference was recognized to lead to under-utilization of cell resources as well as depletion of available channels.
In order to avoid such under-utilization, reuse patterns were adopted in the art wherein different channel sets are assigned to different cells so that callers in adjacent cells tend not to utilize the same channel concurrently. Problems with such reuse patterns, however, include difficulty in creating a cell reuse pattern utilizing channels in such a way as not to have any two cells' use of a channel overlap, as well as limitations on the number of channels available for use in implementing such a reuse pattern.
In a code division multiple access (CDMA) system, the users are separated from one another by using different codes and/or different time delays of the same code while utilizing the same frequency band. Because of this use of the same frequency band, there is a potential, as the system becomes loaded with a number of users, of heavy traffic interference between one user and another limiting the capacity of the system. There are certain problems that are inherent to CDMA networks including interference from one cell to another, since typically every cell reuses the same frequency. The forward link (transmission from a cell site to a subscriber station) at any particular subscriber unit's location may receive interference from a number of cells. Some of those would be desired cells that the subscriber unit would be in handoff with. Others would be cells that the subscriber unit could not be in handoff with, but that would interfere with the signal that the subscriber unit was receiving. An analogous problem happens on the reverse link (transmission from the subscriber station to the cell site), where a cell site receives signals from subscriber units that are within the coverage area of that sector, as well as from subscriber units that are being served by other cells. Interferences of this type limit the capacity of a given sector.
To reduce the interference problems caused by other users in the omni cell 360.degree. configuration, cells have also been broken down into 120.degree. sectors such that each channel available at the cell only communicates in an area of 120.degree. radial coverage about the cell, i.e. sectorized cells. However, a problem with going from the omni cell 360.degree. configuration to the sector system is that, as a result of splitting of the cell into 120.degree. sectors, only a third of the channels are available in each sector. This results in a reduced total call capacity in any particular cell sector as compared to that available in the omni cell 360.degree. configuration. This is because if all of the channels in a particular sector are currently being utilized by users, a channel available in another sector in that same cell may not be available for utilization by a new caller located in the loaded sector. For example, if an omni cell has 60 channels and a sector system is divided into three 120.degree. sectors, each sector only has 20 channels. If in sector 1 there are 20 channels being used and a twenty-first user attempts to gain access, this user will not have access to the cell because of a lack of available channels in the sector. Whereas, in the omni cell 360.degree. configuration, the twenty-first user would have had access to the cell because all channels are by definition potentially available throughout the cell.
Of course one solution might be to add to the total number of channels at the cell. However, this solution is undesirable in that the addition of channels further complicates establishing cell re-use patterns. Furthermore, as the number of channels per sector increases, the possibility of interference events also increases.
Likewise, the addition of channels increases the energy density within the cell and thus reduces the carrier to interference ratio which results in poorer signal quality. For example, in a CDMA system, which is interference limited, additional codes (channels) may be utilized in a sector in order to provide capacity enough to handle calls originating therein, however signal degradation will be experienced. Accordingly, it is preferable to limit the number of codes, and therefore the number of subscriber units serviced, within a sector to a number for which a desired communication quality may be maintained.
It shall be appreciated that loading of sectors is often cyclic or dynamic in nature rather than constant. For example, during certain times of day, such as business commuting times, a particular sector, such as a sector encompassing an urban highway, may service more users than during other times of day. Therefore, during particular times a particular sector or sectors may require increased capacity in order to service all users whereas at other times the cell's capacity might be better utilized when spread more homogeneously throughout the cell's coverage area.
It would, therefore, be advantageous to make more efficient use of cellular capacity by being able to make sectors dynamically shapable in order to provide increased capacity to a particular area within the cell's radiation pattern. This can be done by making more channels potentially available to that particular area, without actually increasing the total number of channels within the cell or the individual cell sectors. Ideally, the shapable sectors will be composed of narrow beams so as to provide a convenient means by which sectors may be sized radially about the cell. Systems implementing such narrow beams are described in U.S. Pat. No. 5,563,610, entitled "NARROW BEAM ANTENNA SYSTEM WITH ANGULAR DIVERSITY," incorporated herein by reference. Management of such a system, including concurrent beam and channel management within a neighborhood of cells, is disclosed in the above referenced co-pending and commonly assigned U.S. Patent application entitled "METHOD AND APPARATUS FOR IMPROVED CONTROL OVER CELLULAR SYSTEMS."
However, in a system providing shapable sectors, it is desirable to provide for the dynamic adjustment of such sectors without detrimentally affecting communications. Specifically, dynamic adjustment of a sector should not result in dropped communications as areas of sector influence are changed. For example, where wireless communication is in progress in a sector which is to have its shape altered, i.e., its area of influence adjusted, it is typically desirable to maintain communication with that subscriber unit throughout reshaping of the sector. Maintaining such communications may require serving the subscriber unit as if it were in the area of influence of the reshaped sector although the new area of influence no longer encompasses the subscriber unit, causing a handoff of the subscriber unit to a different sector having a new area of influence encompassing the subscriber unit, or the like.
A need therefore exits in the art for a system and method for dynamically adjusting the shape of cell sectors to provide for greater trunking efficiency and the ability to serve more users. A further need in the art exists for dynamic transition of the cell sector shapes to be provided in a manner so as to not detrimentally affect communications serviced during the transition.